1. Field of Invention
The present invention relates to a control circuit using piezoelectric ceramic transformer for driving a back-light device and, more specifically, to a control circuit that is able to automatically scan the response frequency of the piezoelectric ceramic transformer and to drive the back-light device at the best resonance frequency.
2. Related Art
From the trend of development, the application of back-light cold-cathode tubes has drawn more and more attention. For example, the personal digital assistant (PDA) and the mobile phone display are manufactured using its features such as lightness, thinness, and smallness. The cold-cathode tube, however, is driven by a high voltage high frequency source converted from a direct current source. The conventional transformer uses multiple turns of coils and has its limitation and drawbacks. More turns of coils are needed to obtain a high turns ratio. There are practical difficulty in manufacturing and the arc problem. Furthermore, conventional coil transformers have such drawbacks as magnet damage, electromagnetic interference, electrical leakage, and temperature rise. Therefore, under the trend of light, thin, and small electronics, conventional coil transformers are unable to fulfill the needs. A transformer using the piezoelectric characteristics has been gradually applied to related fields to solve the problems with conventional transformers.
The piezoelectric ceramic transformer (PZT) is a converter that converts electrical energy into mechanical or photon energy and then converts the mechanical or photon energy back to electrical energy through a coupling effect. If an electrical bias is imposed on one side electrode of the PZT, a mechanical vibration is generated through the piezoelectric characteristics of the material and the other side electrode of the PZT is coupled to generate an electrical voltage. This phenomenon is termed the piezoelectric effect. Common PZT materials include ADP, Rochelle salt, etc. Since the PZT is small and thin, it is widely applied to driving back-light devices, such as the back-light cold-cathode tubes in the PDA or mobile phone displays and ultraviolet (UV) emitter, for years.
It is an object of the present invention to provide a control circuit which controls, through a logic control unit with input from a brightness adjuster control, a piezoelectric ceramic transform to drive back-light devices. The invention further provides a loop circuit for automatically scanning the response frequencies of the piezoelectric ceramic transformer and the back-light device and driving the back-light device at the best resonance frequency.
The invention takes a logic control unit to control the operation of the whole control circuit. Through the parameter setting of the logic control unit and the function of a feedback circuit, the piezoelectric transformer and back-light devices can be adjusted to operate under the best performance. The invention also provides control circuits with single-layer and multilayer piezoelectric ceramic transformers for driving back-light devices.